When a material is subjected to a severely large compressive force so as to be bent and resultantly plastically deformed (i.e. so-called ECAP process), the material is micro-fined in crystal grain size with its strength dramatically improved. With a material of pure titanium as an example, its crystal grain size can be subdivided to about one thousandth so that the strength is improved by leaps and bounds. In particular, the ECAP process allows strength improvement to be achieved without alloying, thus being valuable. For example, a pure metal such as the plastically deformed pure titanium may preferably be used as a material of artificial teeth for use in implant techniques because it scarcely harms the human body.
A conventionally available plastic working method for performing the ECAP process is disclosed in JP 2003-1321 A. This method includes plastic working of a workpiece material by using a mold in which a generally U-shaped passage is formed. More specifically, in this method, the head of a rod-like workpiece material having a cross-sectional shape generally identical to a cross-sectional shape of the passage is inserted into the passage through one opening of the passage, and thereafter an end face of the rod-like workpiece material on its one side opposite to its insertion side is pressed by a plunger or the like with a severely high pressure of several hundreds of tons or so, so that the rod-like workpiece material is passed through a bent portion of the U-shaped passage. During the passage of the workpiece material through the bent portion, the workpiece material is forcedly changed in its extending direction, thereby causing a severe shearing force and an accompanying severe strain to occur to the workpiece material in the passage of the workpiece material through the bent portion. In this way, by imparting severe plastic deformation to the workpiece material, the workpiece material is abruptly micro-fined in crystal grain size, by which material characteristics of the workpiece material are improved.
Unfortunately, with the conventional method described above, since the workpiece material is moved in the U-shaped passage by applying pressurizing force to the end face of the rod-like workpiece material having a cross-sectional shape generally identical to the cross-sectional shape of the U-shaped passage, the workpiece material is laterally strained in proportion to Poisson's ratio by the pressing force, i.e., the workpiece material is stretched perpendicularly to the extending direction of the workpiece material, so that a dynamic frictional force between the workpiece material and the passage wall surface becomes enormously large. This causes the mold to be early worn or damaged, leading to a problem that the mold life is very short.
Also, because of the enormously large dynamic frictional force between the workpiece material and the passage wall surface as described above, the force required to move the workpiece material in the U-shaped passage becomes very large. This causes a problem that an apparatus for plastically deforming the workpiece material or energy cost for driving the apparatus is very large.
Another plastic working method for performing the ECAP process is disclosed in JP 2004-167507 A. This method employs a mold in which an L-shape bent material passage is formed. This mold is a single mold formed by integrally joining two parts together to form an L-shaped passage. In this method, a workpiece material is inserted through one opening of the L-shaped material passage, thereafter an end face of the workpiece material on its one side opposite to its insertion side is pressurized by a pressure device to make the workpiece material extruded from the other opening of the material passage, so that the workpiece material is micro-fined in crystal grain size by the bent portion of the material passage, thereby improving the material characteristics of the workpiece material.
However, with this method also, since the workpiece material is pressurized from its one end, the workpiece material is laterally strained, so that a dynamic frictional force between the workpiece material and the material passage becomes enormously large. This causes such problems as the mold's shorter life and very high cost due to the large pressing force for the plastic deformation of the workpiece material.